1. Field of the Invention
This invention relates to a process for regenerating spent seed materials used in magnetohydrodynamic (MHD) power generation. More particularly, this invention relates to an electrodialysis process for regeneration of spent potassium carbonate seed from mixtures of the carbonate seed and potassium sulfate.
2. Brief Description of the Prior Art
Magnetohydrodynamic (MHD) power generation is based on the direct conversion of heat to electricity by passing a high-temperature, high-velocity, electrically conducting working fluid through a magnetic field. In a coal-fired, open-cycle MHD system (OCMHD), the working fluid is obtained by combusting the coal and seeding the combustion gases with easily-ionizable seed material such as potassium and cesium salt. In a combined cycle power plant with an MHD topping system and a conventional steam turbine bottoming cycle, preheated compressed air and coal are burned in a fuel-rich environment under pressure and at very high temperature in the combustor. The seed is injected and the combustion gas/seed mixture is fed into the MHD channel; there, the interaction of the magnetic field and the ionized plasma results in an induced voltage that is tapped by electrodes, producing DC electrical power, which can then be converted to AC power. Combustion gases then pass through a diffuser, where the kinetic energy is converted to pressure. Exhaust gases leave the MHD generator-diffuser at 2200.degree. to 2300.degree. K. The residual heat must be utilized to produce high-pressure steam and to preheat combustion air. In a typical concept of an MHD steam bottoming plant, the combustion gas is passed through a refractory-lined radiant boiler, where the gas is cooled slowly, to reduce NO.sub.x to an acceptable level before additional air is added and combustion is completed. Combustion gases then flow through a series of heat exchangers, in which the saturated steam produced in the boiler is superheated, air for the primary combustor is heated above 1000 K., and boiler feedwater is heated. Most of the entrained seed is deposited on these heat exchangers as K.sub.2 SO.sub.4, but control devices must be employed to remove the remaining seed material and fly ash particles in the stack gas.
In addition to having higher efficiency than other fossil-fueled power systems, the OCMHD power system has the advantage of a self-contained sulfur-removal capability. Potassium salts, which are the generally preferred seed material, increase the electrical conductivity of the hot combustion gases by thermal ionization, and can remove SO.sub.2 from the gaseous effluent. It has been shown experimentally that sulfur oxide levels produced by the combustion of high sulfur coals can be lowered to environmentally acceptable levels. The spent seed is collected in various downstream components, predominantly as a mixture of water-soluble salts, K.sub.2 CO.sub.3 and K.sub.2 SO.sub.4, contaminated with fly ash and other impurities.
Because of the high cost of seed and the large quantities of seed required, a sulfur removal technique in which the K.sub.2 SO.sub.4 is disposed of and is not reprocessed and reused in MHD plant is possible only if there is an adequate supply of K.sub.2 CO.sub.3 and sufficient demand for K.sub.2 SO.sub.4. At present, it is not economically feasible to consider such a system. Therefore, to exploit the desulfurization capability of the MHD seed material, sufficient quantities of sulfur must be separated from the recovered K.sub.2 SO.sub.4 so that the seed can be reused.
While there is no established seed recovery process, various methods have been proposed for regeneration of spent potassium carbonate seed from coal fired magnetohydrohynamic processes. However, each of these proposed methods suffers from inherent disadvantages. One such method is the Engel-Precht Process, in which the potassium sulfate in the spent seed is reacted with magnesium carbonate in the presence of carbon dioxide to form the magnesium carbonate potassium bicarbonate mixed salt hydrate which, in turn, is treated with magnesium oxide in the presence of water to form magnesium carbonate and regenerated potassium carbonate. There are certain advantages for the process, but the prior art also reveals many unacceptable disadvantages. For example, the reaction to provide potassium carbonate results in the disposal of one mole of magnesium sulfate for each mole of potassium sulfate regenerated. Further, conversion of potassium sulfate is low and potassium losses of approximately 40% can be expected. In addition, water pollution problems result from difficulties in removing magnesium sulfate and potassium sulfate from aqueous solutions because of their high solubilities.
Another prior art process proposed for use in regenerating spent potassium carbonate seed material is the double alkali process. In that process the potassium sulfate contaminant is reacted with calcium hydroxide to form potassium hydroxide and gypsum and the potassium hydroxide is, in turn, reacted with carbon dioxide to form potassium carbonate. In order for this process to proceed, extremely dilute solutions are required which necessitates large expenditures of energy to concentrate the aqueous solutions so that the potassium salts may be recovered.
In the formate process of U.S. Pat. No. 2,030,082, the potassium sulfate impurities are converted to potassium carbonate by reaction with carbon monoxide to form the formate salt which is then treated with oxygen. One disadvantage is that this process is complex and involves the use of complex apparatus because of the necessity that the carbon monoxide is free of carbon dioxide, and that carbon monoxide must be compressed to 30 to 35 atmospheres. A further disadvantage is that carbon monoxide utilization to generate the formate salt is unknown, and the high expense of oil and of natural gas required to generate carbon monoxide gas increases the cost of the process. Moreover, temperature control is critical to potassium salt lost in the double salts, and filtration problems exist because of the tendency of calcium sulfate to form gels.
Another procedure suggested for regeneration of spent seed materials is the PERC Process. In the PERC Process, the potassium sulfate impurities in the spent seed materials are reduced to potassium sulfide, which is then reacted with carbon monoxide and steam to produce potassium carbonate and hydrogen sulfide. While having one advantage in that the PERC Process is carried out in the solid and gaseous phases, it is very inefficient in that carbon monoxide gas utilization is less than three percent.
The aqueous carbonate and modified Tampella processes were developed for use with sodium and not potassium salts. However, it has been suggested that an analogous, modified version of these processes can be used in purifying potassium salts. Basically, in these modified processes potassium sulfate impurities are reacted with carbon at high temperatures to yield potassium sulfide which is reacted with carbon dioxide and water to form potassium carbonate and potassium bicarbonate. One disadvantage of these modified processes, is that they have not been fully modified for use in potassium salt recovery systems. Another disadvantage of these modified processes is that when they are employed in regenerating seed materials used in a magnetohydrodynamic power generating system where coal or coke is used as the reducing agent, potassium silicate and aluminate combustion product removal from potassium carbonate is difficult. A further disadvantage results from the presence of a liquid salt in the reduction vessel which causes serious materials problem.
The Markant Process has also been proposed for use in regenerating seed materials. In that process, potassium sulfate is heated with carbon forming potassium sulfide, which, in turn, is treated with zinc oxide in water to form potassium hydroxide. The potassium carbonate is then obtained by treating the corresponding hydroxide with carbon dioxide and water. One disadvantage of this process is that there are some indications that insufficient conversion of potassium sulfide to potassium hydroxide, and incomplete separation of zinc oxide and potassium salts will be a problem. Further, there are indications that potassium sulfide salts will be lost in the second step of the process, and that further potassium salts will be lost by reaction with impurities in ash constituents when coal or coke are carbon sources in the reduction step.